: The expression pattern of SRC3 will be established during development using the Lac-Z enzyme marker linked to the SRC3 promoter. The role of SRC3 in development will be addressed by examining the structural defects and functional disorders that occur in a recently developed SRC3 knockout mouse model and will include tests for viability, growth, fertility, endocrine regulation and steroid receptor function. The role of SRC3 in mammary gland development, differentiation, and function will be determined in the SRC3 knockout mice. To establish the function of SRC3 in breast cancer initiation and progression, breast cancer will be induced in transgenic knockout mice by tissue specific expression of transgenic oncogenes or treatment with chemical carcinogens. The role of SRC3 in androgen receptor mediated prostate growth and target gene expression will be investigated in the SRC3 knockout mice. It is hoped that these studies will reveal new molecular targets for the diagnosis and treatment of breast cancer.